1 Field ofthe Invention
The present invention relates to a method of forming a protective cover of a pin grid array (to be referred to as a PGA hereinafter) in which an IC chip is mounted on a resin board having contact pins on its lower surface and which serves as an interchangeable memory of a microcomputer or the like.
2 Description of the Prior Art
When a PGA having an IC chip thereon is mounted in an apparatus and then replaced with another PGA, the function of the apparatus can be changed to another function. Therefore, the PGA is widely used to increase the application fields of the apparatus. As a circuit board of the PGA of this application, a ceramic board, resin board, or the like is used. A ceramic board has a good insulating property and thus has high reliability as a product. However, since a wiring pattern is formed on the ceramic board by printing or baking, contraction occurs and it is difficult to increase the number of wiring patterns and to miniaturize a pattern. When the number of wirings is increased, the board size is inevitably increased, resulting in a high price.
A resin board has been recently used, in place of a ceramic board, as the circuit board of a PGA. FIG. 1 shows an example of a resin board wherein a frame 2 is attached to a resin board 1, and a potting resin 3 is dropped in the frame 2, thereby molding an IC chip 4. The IC chip 4 and a terminal pattern 5 printed on the resin board 1 are electrically connected through lead wires 6. Contact pins 7 extend from the lower surface of the resin board 1. Stand pins 9 having steps for height regulation extend from a peripheral portion of the lower surface of the resin board 1 in order to facilitate attachment/detachment with respect to a PGA socket 8. A PGA of this type enables micropatterning at a reasonable cost since the molded resin 3 is in tight contact with the IC chip 4 and a circuit pattern is formed on the resin board 1 by etching. However, its reliability is not satisfactory since the resin has wettability. More specifically, in resin molding by means of potting, a molded portion becomes porous and allows penetration of a moisture, and moisture penetration from an interface A between the resin board 1 and the molded potting resin 3 and from cut side surfaces B of the board typically occurs. In order to solve this humidity problem, a PGA as shown in FIG. 2 is conventionally known. In the PGA shown in FIG. 2, an IC chip 4 is mounted on a resin board 1 and molded by a resin 3, and an entire upper surface of the resultant structure is covered by a metal cap 10. Although the PGA of this type improves moisture penetration to a certain degree, it requires a special manufacturing step for covering the metal cap 10, and moisture still penetrates from a gap between the metal cap 10 and the resin board 1.